1. Field of the Invention
This invention relates to the fabrication of thin film magnetic read/write heads and particularly to a method for forming a DFH (Dynamic Flying Height) slider to achieve high DFH efficiency that is uniform across a disk surface, stable aerodynamics and minimum variations of flying height under a wide range of conditions.
2. Description of the Related Art
FIG. 1 is a highly schematic and simplified illustration showing a read/write head (30) mounted in a DFH slider (1) in operation over a spinning disk (20) in a hard disk drive (HDD). The slider is a ceramic structure with a smooth, planar surface (300), called its ABS (air bearing surface) that is supported over a rapidly spinning disk by aerodynamic forces produced a flow of air between the slider and the disk (25). The slider is attached by a flexible member (26) to an arm (22) called the head gimbals assembly. The height above the disk at which the slider maintains its position while the disk is rotating is called its flying height (80). The rotation of the disk relative to the slider is in the same direction as the airflow, shown by an arrow (25). The edge of the slider into which the disk rotates (200) is called the “leading edge” of the slider, while the opposite edge (100) is called the “trailing edge.” The read/write head, also called a transducer (30) is mounted near the trailing edge of the slider. In a DFH type slider, heating coils (60) are embedded in the slider adjacent to the read/write head. The purpose of these coils is to heat the slider in the region around the read/write head and cause a thermal protrusion of the slider in that region so that the read/write head can be made to approach the surface of the disk more closely. This provides a mechanism for controlling the flying height dynamically, i.e., while the disk is actually in motion. These coils are controlled by external circuitry (58). As will be seen from the discussion below, this thermal control of the flying height can be accompanied by some problems.
FIG. 2, taken substantially from Hashimoto et al., (US Published Patent Application 2007/0058296), shows, in more detail, the surface topography of a commercially available (prior art) slider. Like the slider of FIG. 1, this slider is provided with thermal control of its flying height (i.e., it is a dynamic flying height slider) for use in a hard disk drive (HDD) as discussed above. Some features of this slider are worth noting.
The slider (1) has a pre-patterned air-bearing surface (ABS) plane (3) that faces the rotating disk during HDD operation. Although the ABS plane is substantially planar, as indicated in FIG. 1, FIG. 2 shows that it actually has a patterned topography, including outward projecting pads (10), (12), rails (8) (9) and incised grooves (11), (15) which extend into and out of the body of the slider vertically away from the surface plane. This slider, as the one in FIG. 1, is typically mounted on the distal end of a head gimbal assembly (HGA) (not shown here) that is activated by an electromechanical mechanism and control circuitry to position the head at various positions along the magnetic tracks on the disk (not shown).
The leading edge of this slider is labeled (2), the opposite trailing edge, which contains the read/write head, is labeled (4). The read/write head (17) is encapsulated within the slider at its trailing edge in a “rear pad” (12) and, as we shall see below, in the dynamic flying height (DFH) type slider, such as illustrated in FIG. 1, the read/write head is also surrounded by, or adjacent to, embedded heating elements (not shown here). The slider topography also includes airflow grooves (11) that are etched into the slider surface to provide an enhanced aerodynamic performance. The aerodynamics of the slider motion lifts the leading edge higher above the rotating disk surface than the trailing edge.
For a typical disk drive (approx. 200 Gbyte/platter) the flying height distance (80) between the magnetic head and the media is between approximately 10 nm (nanometers). It is essential that the sliders fly with aerodynamic stability over the disk surfaces during reading and writing.
Currently, the distance between the slider and the media has been pushed to as low as 5 nm during read processes via the dynamic flying height (DFH) technology, which is exemplified by the slider in FIG. 2. This technology is also described, for example, in Hashimoto et al. (US Published Patent Application 2006/0139810), Kato et al, (U.S. Pat. No. 7,164,555), Payne et al. (US Published Patent Application 2006/0092570), Riddering et al. and (U.S. Pat. No. 7,068,457). Pust et al. (US Published Patent Application 2003/0128469) shows bonding pads and studs that reduce thermal deformation.
As already noted, DFH technology achieves local flying height reduction by applying a voltage to a heater embedded in the slider body. Heat supplied by the heater increases the temperature of the slider in the heater's vicinity and this increase in temperature, in turn, causes the surface of the slider to protrude as a result of thermal expansion of the surrounding material. In principle, this protrusion will bring the read/write head closer to the disk surface, thus reducing the flying height and allowing for greater resolution in the read/write process.
During the resulting temperature induced protrusion process, however, the slider will be pushed back by a protrusion-induced increase in the air pressure acting on the slider due to the squeezed layer of air within the head/disk interface. This additional air pressure acts counter to the desired flying height reduction that the heater-induced slider protrusion is meant to produce. Thus it is highly desirable to provide a method of decreasing flying height by a thermal process, while not allowing that very decrease to counter the desired effect.
In DFH technology, the heater is turned on only when a read or write operation is called for. This substantially improves the reliability of the head/disk interaction for the following reasons: 1) the magnetic head does not have to constantly fly at low flying heights; 2) the magnitude of flying height reduction can be made to depend on the environmental conditions, for example a smaller height reduction is required at high temperatures and high altitudes; 3) the flying height minimum point is always at the heater area, the other areas of potential contact are always higher and, therefore, the opportunities for contact are reduced; 4) even if there is a contact at the heater area, the contact force is smaller due to the reduced area of contact and, therefore, there is less chance of creating head modulation and related read/write failure.
The various processes cited above have created the following meaningful challenges for slider design in DFH applications. The following three challenges, denoted A, B, and C, are associated with the design of the air bearing surface.
A. Very High Pressure is Applied on the Heated Area of the Slider.
This produces what is called “pushback” or ABS (air-bearing surface) compensation, which is the counterproductive effect of preventing the local deformations of the slider body that are required to produce good DFH efficiency. The DFH efficiency is defined as the ratio of the actual flying height reduction to the slider body protrusion height (or, equivalently, to heater power). If the protrusion produced by a given input of heater power is negated by the added pressure pushing the slider away from the disk surface, then the effects have canceled each other and more heater power is required to accomplish a given flying height reduction. One approach to mitigating this problem is, therefore, to simply apply higher power to the heater. Unfortunately, over long term operation this can either degrade the reader performance or cause excessive power consumption or both. Alternatively, to further improve the DFH efficiency of air bearing sliders for DFH applications, traditional designs attempt to reduce the pressure acting on the entire slider body. This approach sacrifices the flying height sigma, i.e., the tight control over statistical variations in flying height for a set of sliders.
B. Large Disk Distortion at the Inner Radius.
Disks usually have large distortions under disk clamping forces. This produces an undulating disk surface and a large flying height variation between the slider and the disk across the disk surface. This distortion is more pronounced at the inner diameter (ID) than the outer diameter (OD). This creates yet another challenge to achieving a stable flying height across the entire disk surface. Lowering the pressure at the area where the magnetic sensor is carried will significantly increase the sensitivity to local disk distortions at the inner radius.
C. DFH Variations Across the Disk Radius.
For traditional air bearing surface designs the DFH efficiency usually varies across the disk radius. At the inner diameter (ID) the DFH efficiency is usually greater than it is at the middle of the disk (MD) or at the outer portion of the disk (OD). This is because air flow at the ID is not as strong as it is at the MD and OD. The pushback of the slider due to air squeezing at the ID is smaller, therefore, the DFH efficiency is higher at the ID.
The following three challenges, denoted A, B, and C, are a result of the specific requirements of consumer electronics.
A. Temperature Requirements.
Consumer electronics devices are required to operate within the large range of temperatures between −20° C. and +80° C. The flying height between the magnetic head and the media surfaces can change due to mechanical changes in the system resulting from the temperature variations. For example, the static pitch altitude (PSA) of the head gimbal assembly (HGA) can change and, additionally, the temperature variations can create changes in the shape of the slider crown. It is therefore desirable that an ABS design can be able to compensate for flying height changes due to changes in the slider shape.
B. Altitude Requirement.
Consumer electronics devices are usually required to operate at an altitude of 10,000 ft. Since the air density at such an altitude is much lower than that at sea level, the high altitude has a direct impact on the flying height between the magnetic head and the media. It is therefore desirable to have a slider ABS design that minimizes the flying height changes due to high altitude.
C. Power Requirements.
Consumer electronics devices also have a limitation on the amount of power that can be used during drive operations. Higher DFH efficiency will reduce the power necessary to achieve the necessary flying height to read and write.
Different approaches have been suggested for achieving higher DFH efficiency. One approach is via ABS design. Hashimoto et al., cited above with reference to FIG. 2 and Hashimoto et al. also in U.S. Patent Application 2006/0139810, describes an isolated ABS pad ((12) in FIG. 2) for achieving flying height control by DFH. The operation of the pad is to reduce the push back effect caused by protrusion by moving the pressure peak on the ABS from the pad itself to a position on the ABS surrounding the pad.
Since the ABS pressure at such an isolated pad is small, a large deformation/protrusion can be achieved at low heater power, thereby producing a high DFH efficiency. However, this design could lead to instability or modulation of the head due to the separation between the read/write pad and the main air bearing pressure center. For that reason and others, the present invention proposes a bridged area to provide pushback on the read/write head area.
It is the view of the present inventors that none of the aforementioned approaches will achieve the stable and controllable DFH slider dynamics and improved DFH efficiency of the present invention as defined by the following objects and method of achieving them.